The present invention relates to a method and apparatus for measuring various optical properties of materials, including the properties of absorption, optical rotation and depolarization. While the present invention has been constructed to be used as a detector for chromatographic separations, the device may be employed in situations where measurement of these optical properties is desired for a generally transparent, fluid material.
Various polarimeters have been developed in the past, and many of these have been based on a common physical principle that, when plane-polarized light is passed through a material, the plane of polarization of the light is rotated. As is known, nonpolarized light has random orientations, that is, it is composed of light comprised of tranverse waves having all orientations around the axis of propagation, while plane-polarized light is composed of transverse waves having a single orientation with respect to this axis of propagation. Certain substances have the ability to rotate plane-polarized light as it passes through the substance. That is, these substances rotate the orientation of the polarized light about the axis of propagation as the light passes through the substance. The amount of this rotation is related to the substance or material rotating the light, the concentration of the substance, and the path length of light through the substance. Specifically, the specific rotation of the substance is: ##EQU1## where: ##EQU2## Implied in this equation is that specific rotation is dependent upon the temperature of the substance and the wave length of the light used to measure the rotation. By convention, counterclockwise rotation about the axis of propagation is given a negative sign, and clockwise rotation is given a positive sign. Standard measurements are normally conducted at 25.degree. C. with the sodium d line of 5891 angstroms in order to eliminate the variables of temperature and wavelength.
In most prior art devices, optical rotation is observed by passing the rotated light through an analyzer in the form of a polarizing filter, such as a prism whose optical axis is oriented at an angle with respect to the original plane of polarization. The intensity of the light is diminished by an amount dependent upon the angle of the plane of polarization after rotation and the optic axis of the analyzer. In many prior art devices, the analyzer is oriented in a crossed configuration, i.e. 90.degree., from the orientation of the plane-polarized lights in order to minimize the amount of light which would pass through the analyzer and reach a detector and therefore the intensity of light at the detector. A sample cell containing the material to be tested is placed between the polarizer and the analyzer so that any rotation of the polarized light as a result of the material will be evidenced by an increase in the intensity of light reaching the detector. Typically, these devices measure the optical rotation by mechanically rotating either the analyzer or the polarizer so that the intensity of light at the detector is reduced to a minimum value or null position. The amount of rotation of the polarizer or analyzer is then exactly equal to the rotation property [.alpha.] of the sample.
An alternative to mechanical rotation found in the prior art is the rotation twisting of the light by the use of a Faraday rotator. A Faraday rotator is simply a material (optically active or not) that is immersed in a magnetic field along the axis of the transmitted light. The light beam's plane of polarization is rotated proportionally to the strength of the magnetic field component of the Faraday rotator and the length of the light path therethrough.
Examples of these various prior art devices may be found in U.S. Pat. No. 3,510,226 issued May 5, 1970 to Kerry which shows a Faraday coil being used as a compensator for rotation caused by a sample to be measured. U.S. No. Pat. No. 3,361,027 issued Jan. 2, 1968 to Kaye teaches rotation of the polarizer to compensate for the angle of rotation of the sample material while U.S. Pat. No. 4,306,809 issued Dec. 22, 1981 to Azzam teached the use of measuring the rotation of light by means of quarter wave plates.
It is also possible, however, to measure the rotation of the polarized light through the medium by monitoring the intensity of the light at the detector and utilizing this intensity as a basis for calculating the amount of optical rotation. In T. Crumpler et al, "Simple Photoelectric Polarimeter", 27 Analytical Chemistry #10 (1955), the polarizer and analyzer filters are oriented at a fixed angle of 45.degree. with respect to one another. This angle provides for the maximum change in intensity at the detector per unit rotation of the light. Inherent limitations of this system were improved upon in various devices wherein a polarized beam was split, either before or after it is passed through a sample cell, and then is subjected to two analyzer polarizing filters oriented at +45.degree. and -45.degree. with respect to the orientation of the first polarizing element. Thus, any optical rotation caused by the sample material would cause an increase in intensity passing through one analyzer while a decrease in intensity in the other analyzer.
Despite these improvements in polarimeters, there still remained a need to provide a method and apparatus for overcoming inherent design limitations of these prior art devices. More particularly, the need remained for an optical measuring device which was able to measure various optical properties of a substance, including the optical rotation, the depolarization, and absorption characteristics of a sample material.